Band-pass control means for radio sets



A 8 6,' 940.' J. F. SILVER 2,210,

BAND PAS CONTROL MEANS FOR RADIO SET3 Filed Sept. 12, 1938 M1& M2 1-4 (BROAD) M1 3 M2 1.0 M A? M; 0.7 (SHARP),

RESPONSE FREQUENCY INVENTOR,

rlbhw E 6/4 l/EIE.

BY MQ M \TTORNEYS.

Patented Aug. 6, 1940 UNITED STATES PATENT poration of Ohio Applicationseptember'12,1938. Serial No. 229,467

10 Claims. (01. re- 171) In the use of present day 'superheterodyne radio sets, the great numberof high power transmitting stations in operation requires extremely sharp selectivity, in order to separate them. But extremely sharp selectivity is not consonant with higher fidelity audio reception, as is understood in the art.

A further difiiculty presents itself in this, that if the intermediate frequency'amplifier has extremely sharp selectivity, the receiver becomes very critical in tuning. Not only does this make the receiver difficult to tune manually, but any automatic tuning system which is the trend today appears to be toward the use of some sort of automatic or semi autom'atic tuning system), must be capable of tuning with extreme accuracy. I

The conventional intermediate frequency amplifier consists of one or more stages of ampli- 2 'o' fication and usually consists of a vacuum tube and two tuned circuits inductively coupled as is well understood. The two tuned circuits are tuned to resonate at the desired intermediate frequency or I. F. carrier. By varying the mutual 25. inductance between the tuned circuits, the band pass width may be varied considerably. But the requirement for selectivity cannot be met with the ordinary two circuit transformers because of the conflicting requirements noted ,above.

3.011 The best that canbe accomplished is a compromise.

Three circuit intermediate frequency transe formers have been suggested, and these effect an improvement in the bandpass characteris- 35. tics. In such amplifier circuits the transformer consists of a primary, a secondary andtertiary winding, each being tuned. By varying themutual inductance considerable modification of. the band. pass characteristics can again be. secured. The ideal intermediate frequency amplifier would be a bandpass filter, having noattenuation over the pass band and infinite attenuation at the high and low cut-off frequencies. The radio receiver would be tuned -to -mid-fre- 45 quency half way between thecut-off-frequencies.

With the present day frequency allocations of transmitters (which allow stationsto be located every-tenkilocycles 'in the spectrum) the cut-off 50 frequencies would be only ten thousand cycles apart. Thus, with the ideal amplifier, not "only would high selectivity be achieved, but the receiver would be capable of receiving and translating without distortion audio frequencies 'of zero to five'thousand cycles. Needlessto say,

employed (and these ideal tice.

It is an conditions. are not attained in pracobjeet of my invention to provide 'a novel intermediate frequency amplifier, having means for changing from broad to sharptuning, in which the band pass characteristics "more clearly approach those of the ideal'amplifier. It

is an objec termediate t of my invention-.to-provide 'anim frequency amplifienhaving a number of advantages over the amplifiers hitherto devised. It is a further object'of-my invention to provide a system in which the ratio of the pass band attenuated 2:1 or less to that attenuated 10': l o

r'less is better than can be obtained with two circuit transformers (the Q of all circuits being Consideri the same). ng further the two circuit transformer system, one of its disadvantages is that when the coils are over coupled, the points of maximum response are located to either side of the mid point.

Hence; if the receiver were tuned for maximum response, it Would'have to be tuned so that the intermediate frequency carrier is located on one of thehigh points. This would tendto give single side band reception and in some casesa distorted detector output. Also any regeneration would tend to occur at one of the high gain points, and the response curve would be very assyminetri cal. Thus any change in the coupling to alter the band pass characteristic of the amplifier from selective to broad or vice versa usu-' ally requires that the set be retuned each time the couplin invention t g is changed. It is an object of my 0 provide'an intermediate frequency amplifierin whichthe selectivity may be changed from one passing only a very narrow band of frequencies to one passing practically all of the transmitted side bands, and which will not detune the receiver when the degree of selectivity ischanged. ferred emb plished' ma operated fr and other The change of selectivity in the preocliment of my invention is accomnu'ally by throwing a simple switch om the'front of the radio set. These objects of my invention which will be set forth hereinafter or will be apparent to one skilled. fications, I

in the art upon reading thesespeciaccomplish by that certain construction and arrangement of parts of which Ishall now describe a preferred embodiment.

Refer ence may be made to the drawing wherein:

Figure 1 is a circuit diagram intermediate frequency stage. q

Figure 2 is a graph, showing response plotted against fre apparatus.

showing my novel' quency "for diiferent settings of the In the apparatus and circuit of my invention, I employ a transformer with primary, secondary,

v and tertiary windings which are coupled to certain of the other windings. This gives me a construction wherein the mutual inductances between the coils can be made either to oppose or to reenforce each other, so as to give loosely coupled or overcoupled conditions in the transformer without disturbing the tuning of the radio set.

In Figure 1, I have indicated the input tube for the intermediate frequency amplifier at l. The plate of this tube is connected to the high side of a primary winding 2 of the intermediate frequency transformer. A condenser 3 forms a closed resonant circuit with the primary. The condenser is variable, of course, for tuning this circuit. The low end of the primary 2 may be grounded through a condenser 5 as at 6. The positive pole of the B source (so marked) is connected to the circuit. The transformer has a tertiary winding 1, one 'end of which is grounded as at 8 through a variable condenser 9, the other end of the tertiary winding is brought out to the center tap of a coil marked II. The ends of this coil are connected with contact members l2 and I3 respectively of one portion of a double pole, double throw switch [4, the movable portions of which are grounded as at l5. This completes the circuit of the tertiary winding.

The transformer also has a secondary winding it, one end of which is connected through a variable condenser l'! with the center tap of a winding l8. to the contacts [9 and 2B of the other part of the double pole double throw switch I4. One of these connections may have a resistance 2! in series with it. The other end of the secondary winding [6 is grounded as at 23 through a condenser 24. An automatic volume control connection (so marked) may be made to the secondary as shown.

The secondary circuit is brought out to the grid of the output tube 25.

The transformer, so far as its primary, secondary and tertiary windings are concerned, is of the usual type. The coils II and I8 consist merely of a few coupling turns, center tapped and wound on the low ends of the primary and tertiary windings 2 and I of the transformer.

The intermediate frequency stage therefore comprises primary, secondary and tertiary tuned circuits; butthe tertiary and secondary circuits include windings related respectively to the primary and tertiary windings of the transformer. There is, of course, an inductive coupling between the primary and tertiary windings, which I have indicated at M1 in the drawing. Likewise, there is an inductive coupling between the tertiary winding and the secondary winding which I have indicated in the drawing at M2. There is also inductive coupling respectively between the coil II and the primary 2, and between the coil [8, and the tertiary winding 1, depending, of course, upon the direction of the voltage in the coils H and I8. With the switch in the position shown in Figure l, the coupling between coils H and 2 aids the coupling M1, while the coupling between coils l8 and 1 aids the coupling M2, so that the transformer is overcoupled and the width of the pass band is increased. When the switch is thrown in the opposite direction, the coupling between coils 2 and H and 1 and I3 respectively opposes M1 and M2, thus narrowing the pass band width. In setting up the apparatus, M1 and M2 are adjusted to approximately critical coupling. The number of turns in the coils II and I8 is The ends of this winding are brought out kept small so that the effect of stray capacity coupling is negligible.

In Figure 2, I have plotted response against frequency for difierent conditions of coupling in my transformer at 26, 21 and 28. I have marked the inter-coil couplings in connection with the several curves. It may be noted that in the over-coupled condition (e. g., curve 28) three points of peak response are obtained; but the greatest of these three appears on resonance, that is to say, on the line of the mid-frequency 1% for which the intermediate frequency amplifier is tuned. This keeps the signal properly tuned in for maximum strength. Also, most tendency to regenerate occurs near this mid-frequency peak, since it is the point of maximum gain in the system. The curve 28 shows the response for broad tuning, if the system does not contain the resistor 2|. The resistor 2! (of a few ohms) where used, has the effect of reducing the height of the mid-frequency response peak as indicated in dotted lines in Figure 2. The resistor is in the circuit'only when the switch i4 is thrown to the broad position. Thus a flatter top on the broad band pass curve is obtained; yet the gain on resonance is still slightly higher than the gain at any other position in the band.

Having thus described my invention, what I claim as new and desire to. secure by Letters Patent, is: V

1. An intermediate frequency amplifier having input and output tubes and a transformer having primary, secondary and tertiary tuned windings, with inductive coupling between the primary and tertiary and between the tertiary and secondary, the said inductive coupling being adjusted to approximately critical coupling, and means for varying the coupling between said windings respectively to vary the width of the pass band, the said means comprising a tapped coil in the circuit of said tertiary windingand. inductively coupled to said primary, and a tapped coil in the circuit of said secondary winding and inductively coupled to said tertiary winding, and means for causing the couplings due to said last mentioned tapped coils to assist or oppose the inductive coupling between said first mentioned windings.

2. An intermediate. frequency amplifier having a transformer with primary, secondary and tertiary windings adjusted to approximately critical coupling and tuning means in connection with each of said windings, additional windings inductively coupled with two of said first mentioned windings and means for applying voltage derived from certain of said first mentioned windings to said last mentioned windings and means for reversing the phase of this voltage.

3. An intermediate frequency amplifier system,

comprising an input tube. a transformer having primary, secondary, and tertiary windings, a connection between said primary winding and the plate of said input tube, an output tube, a connection between the secondary winding of said transformer and the grid of said output tube, and 7 means for varying the band pass characteristics of said intermediate frequency amplifier, comprising means for causing the mutual inductances between the primary winding and the tertiary winding and between the tertiary windingand the secondary winding, alternatively to be simultaneously opposed and reenforced.

4. An intermediate frequency amplifier system, comprising an input tube, a transformer having primary, secondary, and tertiary windings, a connection between said primary winding and l ances between the the plate of said input tube, an output tube, a connection between the secondary winding of said transformer and the grid of said output tube, and means for varying the band pass characteristics of said intermediate frequency amplifier, comprising means for causing the mutual inductprimary winding and the tertiary winding and between the tertiary winding and the secondary winding, alternatively to be opposed and reenforced, and switching means for selecting the effect desired.

5. In an intermediate frequency amplifier a transformer having primary, secondary, and tertiary windings, additional windings on said primary and tertiary windings respectively, said last mentioned windings having center taps, and a connection between said tertiary winding and the center tap of the coil on said primary winding and a connection between the secondary winding and the center tap of the coil on said tertiary Winding, and switch means in connection with the end terminals of said last mentioned coils for completing the circuits of said tertiary and secondary windings therethrough.

6. An intermediate frequency amplifier comprising input and output tubes and a transformer having primary, secondary, and tertiary windings, additional coils comprising but a few turns and located respectively on said primary and said tertiary windings, said additional coils being center tapped, a closed circuit for said primary winding containing a variable capacity, a circuit for said tertiary winding containing a variable capacity and comprising a connection to the center tap of said coil on said primary winding, a circuit for said secondary Winding comprising a variable capacity and a connection to the center tap of said coil on said tertiary winding, a double pole double throw switch, a connection between said switch and ground and connections between the end terminals contact members on said switch.

7. An intermediate frequency amplifier comprising input and output tubes and a transformer having primary, secondary, and tertiary wind ings, additional coils comprising but a few turns and located respectively on said primary and said tertiary windings, said additional coils being center tapped, a closed circuit for said primary winding containing a variable capacity, a circuit for said tertiary winding containing a variable capacity and comprising a connection to the center tap of said coil on said primary winding, a circuit for said secondary winding comprising a variable capacity and a connection to the center tap of said coil on said tertiary winding, a double pole double throw switch, a connection between said switch and ground and connections between the end terminals of said coils and contact members on said switch, the operation of said switch being such as to cause the voltage in said additional coils to change its phase with respect to the voltage in the windings with which said coils are associated.

8. An intermediate frequency amplifier comof said coils and prising input and output tubes and a transformer having primary, secondary, and tertiary windings, additional coils comprising but a few turns and located respectively on said primary and said tertiary windings, said additional coils being center tapped, a closed circuit for said primary winding containing a variable capacity, a circuit for said tertiary winding containing a variable capacity and comprising a connection to the center tap of said coil on said primary winding, a circuit for said secondary winding comprising a variable capacity and a connection to the center tap of said coil on said tertiary winding, a double pole double throw switch, a connection between said switch and ground and connections between the end terminals of said coils and contact members on said switch, the operation of said switch being such as to cause the voltage in said additional coils to change its phase with respect to the voltage in the windings with which said coils are associated, and a resistance in oneof the connections between the additional coil on said tertiary winding and said switch.

9. An intermediate frequency amplifier including a transformer having primary, secondary, and tertiary windings and circuits therefor, each of said circuits being tunable, there being mutual inductance between said primary and tertiary windings and between said tertiary and secondary windings respectively, said apparatus being adjusted to approximately critical coupling, and means for varying the band pass width of the said amplifier, comprising inductive means for feeding voltage from the primary to the tertiary windings and from the tertiary to the secondary windings respectively, and switching means for changing the phase of the voltage so induced so that it alternatively aids and bucks the voltage induced by said mutual inductance between primary and tertiary and between tertiary and secondary windings respectively.

10. An intermediate frequency amplifier including a transformer having primary, secondary, and tertiary windings and circuits therefor, each of said circuits being tunable, there being mutual inductance between said primary and tertiary windings and between said tertiary and secondary windings respectively, said apparatus being adjusted to approximately critical coupling, and means for varying the band pass width of the said amplifier, comprising inductive means for feeding voltage from the primary to the tertiary windings and from the tertiary to the secondary windings respectively, and switching means for changing the phase of the voltage so induced so that it alternatively aids and bucks the voltage induced by said mutual inductance between primary and tertiary and between tertiary and secondary windings respectively, and a resistance in the circuit of said switch means so connected as to be in the circuit of said secondary winding when said secondary winding is connected to second of said inductive means so that it is aiding first said inductive means.

JOHN F. SILVER. 

